Relative Excitation Features for Speech Recognition

ABSTRACT

Relative Excitation Features, in all conditions, are far superior to conventional acoustic features like Mel-Frequency Cepstrum (MFC) and Perceptual Linear Prediction (PLP), and provide much more speaker-independence, channel-independence, and noise-immunity. 
     Relative Excitation features are radically different than conventional acoustic features. Relative Excitation method doesn&#39;t try to model the speech-production or vocal tract shape, doesn&#39;t try to do deconvolution, and doesn&#39;t utilize LP (Linear Prediction) and Cepstrum techniques. This new feature set is completely related to human hearing. The present invention is inspired by the fact that human auditory perception analyzes and tracks the relations between spectral frequency component amplitudes and the “Relative Excitation” name implies relative excitation levels of human auditory neurons. 
     Described herein is a major breakthrough for explaining and simulating the human auditory perception and its robustness.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 14/782,799, filed on Oct. 7, 2015 and entitled,“Relative Excitation Features for Speech Recognition” which is aNational Stage Filing under 35 U.S.C. 371 of PCT/TR2014/000035 filed inthe Turkish Patent and Trademark Office on Feb. 17, 2014, which claimspriority to Turkish patent application 2013/04371 filed in the TurkishPatent Office Apr. 11, 2013, each of which are incorporated herein byreference in its entirety.

This invention is related to acoustic feature extraction for speechrecognition and other audio processing applications.

Acoustic feature extraction has critical importance for speechrecognition accuracy. But existing techniques are sub-optimal, notimmune to noise, and too sensitive to channel or speaker variation.

Today almost all of the parametric representations of speech depend onCepstrum or LP (Linear Prediction) techniques. The most commonly usedacoustic feature representation is MFCC (Mel-Frequency CepstralCoefficients). Another common acoustic feature representation is PLP(Perceptual Linear Prediction). PLP was proposed by Hynek Hermansky“Perceptual linear predictive (PLP) analysis of speech,” 1990. MFC andPLP methods both produce cepstral coefficients.

Through more than 30 years, thousands of research studies have beenconducted to find out a better parametric representation of speech. Somevariations of Cepstrum techniques have emerged, like Minimum VarianceDistortionless Response MFCC (MVDR-MFCC), Gammatone CepstralCoefficients (GTCC), Power-Normalized Cepstral coefficients (PNCC),Zero-Crossings with Peak Amplitude (ZCPA), Perceptual harmonic cepstralcoefficients (PNCC), etc. All of these methods depend on Cepstralrepresentation of speech. Most of them provide some accuracy improvementover standard MFC method for noisy conditions, but there is no ornegligible improvement for clean speech compared to standard MFC method.A better parametric representation of the acoustic signal is needed.

The present invention discloses a novel and radically different methodfor producing superior acoustic features over PLP, MFC and other similarfeatures. We named this revolutionary new method as “RelativeExcitation”, considering the fact that human auditory perceptionanalyzes and tracks the relations between spectral frequency componentamplitudes. The “Relative Excitation” name is produced from an analogybased on relations between spectral frequency component amplitudes orrelative excitation levels of auditory nerve in human cochlea.

Generally speaking, human perception is relative, and it is sensitive tothe differences. For example, we can easily sense a weak light source indarkness. Darkness provides a low stimulation to neurons on the retinaand the light source causes relatively high simulation, so we can easilysense a weak light source in the darkness, but we may not notice orsense the same light source in the day light. A similar stimulationmodel is true for human auditory. We know the tone masking properties ofhearing very well. The tone masking features of human audition forms animportant evidence for the relative perception of human hearing with atonotopic mapping. Relative Excitation coefficients excellently simulatethe tonotopic and relative perception of human auditory sense.

Relative Excitation features are radically different than PLP and MFCfeatures. Relative Excitation method doesn't try to model thespeech-production or vocal tract shape, or doesn't try to dodeconvolution and doesn't use linear prediction or cepstralrepresentation in contrast to PLP and MFCC. This new feature set iscompletely related to human hearing. Trying to model the speechproduction system or trying to do deconvolution is not an optimalapproach for speech recognition, because there are endless combinationsof articulatory organs' positions and movements. Moreover, the effect ofenvironment and channel increase the complexity of the system that is tobe modeled.

PLP and MFC or other cepstrum based methods have negative propertieslike strong channel-dependence and speaker-dependence or too muchsensitivity to noise. Generally, for these methods, a small change inthe system causes significant unwanted changes of the calculatedcoefficients (Please see FIGS. 6a and 6b ). For example, the cosinetransformation used for calculating the cepstral coefficients causessome portion of noise to affect all of the coefficients. Differentchannels (microphones, electronics, compression techniques, etc.) havedifferent frequency responses. Noise and channel frequency responsevariations have too much negative influence on the coefficients of thesemethods. Utilizing these conventional methods as an acoustic front endfor speech recognition systems;

-   -   increases training and recognition data mismatch,    -   increases the amount of acoustic data needed for healthy        training,    -   increases the variance of trained models, resulting with coarse        models.

As an attempt to mimic human auditory system, PLP, MFC or otherconventional methods use some techniques like time-domain pre-emphasis,weighting according to the equal-loudness contour over the frequencyspectrum, computing the log-magnitude or cubic root representation ofthe spectral amplitudes, and other spectral weighting or normalizationtechniques. We see these techniques as a struggle to heal theproblematic nature of cepstrum for speech recognition.

Advantageous Effects of the Invention

Relative Excitation coefficients are superior over MFC and PLPcoefficients at all conditions. Relative Excitation coefficients provideup to 25% relative error reduction compared to MFCC or PLP for cleanspeech. Error reduction can reach up to 60% at noisy conditions.

Relative Excitation coefficients have very strong noise-immunity,channel-independence, and speaker-independence properties compared toMFC and PLP.

Although the Relative Excitation method is superior, it requires muchless computation for estimating acoustic features compared to MFCC andPLP. There is no need for pre-emphasis, equal-loudness weighting,log-amplitude, linear prediction, or DCT (Discrete Cosine Transform) ofthe cepstrum technique.

Relative Excitation coefficients are completely compatible with cepstrumfeature vectors for statistical processing such as distance measurement,mean and variance normalization, LDA, PCA, MLLR, fMLLR, etc. So RelativeExcitation coefficients can easily replace cepstral coefficients.

Just like Cepstrum, Relative Excitation Method uses a predetermineduniform feature extraction structure which is not dependent on thesignal, and produces coefficients that can represent any audible signalincluding human speech.

Relative Excitation coefficients reduce training data and recognitiondata mismatch. It is possible to train much more healthy Hidden MarkovModels with much less variance.

Relative Excitation coefficients excellently simulate the physicallimits of basilar membrane, human hearing range and sensitivity.

There is a trend towards training LVCSR (Large Vocabulary ContinuousSpeech Recognition) systems on a large amount of found data. Found datais recorded from spontaneous speech without control of the acousticconditions, so found data normally contains multiple acousticconditions, such as environmental noise or spectral deformation causedby the variation of the channel frequency response. If noisy speech isused for training, cepstral coefficients deteriorate acoustic models. Incontrast to cepstral coefficients, some degree of noise seems to bebeneficial for the acoustic models trained with Relative Excitationcoefficients. This property of Relative Excitation coefficients makesRelative Excitation method excellent for using large amount of founddata to train the acoustic models of a speech recognition system.Training LVCSR systems with found spontaneous speech data is veryimportant to get the speech recognition technology closer to the humanrecognition performance.

Many attempts have made been by scientists to exploit the parametricformant representation in speech recognition applications, becauseutilizing formant information in speech recognition may providesubstantial benefits like robustness against additive noise. But, it isnot possible to represent all speech sounds, such as nasals andfricatives in terms of formant frequencies, and additionally, existingformant estimation algorithms are not reliable enough to be used in aspeech recognition system. Unreliable and inconsistent parameter valuesare not suitable for speech recognition, since algorithms are designedto deal with consistent feature vectors of a fixed length that bear thesame measurement information in each element. Until now, there hasn'tbeen a satisfying solution for incorporating formant information inspeech recognition. Relative excitation coefficients are suitable forany signal type and bear the formant information, if exists, without anyinconsistency at the feature vector. Relative Excitation method isdesigned to capture the amplitude differences of spectral regions, thespectral valleys, peaks or formants. When a Relative Excitationcoefficient represents the relation between two spectral components, oneof which is located close to a formant frequency, the coefficient'svalue depends on the relative amplitude of the formant.

In addition to all these benefits, the present invention introduces avery strong novel noise handling technique that resembles the spectralfocusing and ignorance capability of human hearing.

Relative Excitation method opens a door to a new realm that has endlesspossibilities and it is a major breakthrough for understanding andsimulating human auditory perception.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows Relative Excitation feature extraction method.

FIG. 2 compares PLP, MFC and Relative Excitation methods.

FIG. 3 shows sinusoidal filters of main bands of Relative Excitationmethod.

FIG. 4 shows sinusoidal filters of a main band and its comparison band.

FIG. 5a shows spectral amplitudes of a frame selected from a cleanutterance.

FIG. 5b shows spectral amplitudes of the same frame from white noiseadded version of the same utterance.

FIG. 5c shows logarithmic representation of 5 b.

FIG. 6a shows MFC coefficients of a vowel sound frame. The amplitudespectrum of this frame is drawn on FIG. 5 a.

FIG. 6b shows MFC coefficients of the noisy version of the sound frame.The amplitude spectrum of this frame is drawn on FIG. 5b . Pleasecompare FIGS. 6a and 6b , and notice high variation of coefficientswhich shows the weakness of MFCC against noise.

FIG. 6c shows Relative Excitation Coefficients of the clean vowel soundframe which's amplitude spectrum is drawn on FIG. 5 a.

FIG. 6d shows Relative Excitation Coefficients of the noisy version ofthe same sound frame. The amplitude spectrum of this frame is drawn onFIG. 5b . Please compare FIGS. 6c and 6d , and notice the similarity ofcoefficients that shows the robustness of Relative Excitationcoefficients against noise. FIGS. 6a-6d compare MFC and RelativeExcitation coefficients for noise-immunity. Please notice that all ofthe MFC coefficients have large deviation in contrast to RelativeExcitation coeffs. Deviations of the last four Relative Excitationcoefficients (11-12-13-14) are high and this is normal and very useful.Please read the noise-immunity section for more information on thesegraphics.

FIG. 7a compares WER (Word Error Rate) of Relative Excitationcoefficients and MFCC for clean speech (found speech data that may havenon disturbing or mild noise). Relative Excitation coefficients provides23.4% relative error reduction in this test.

FIG. 7b compares WER of Relative Excitation coefficients and MFCC fornoisy speech (significant or disturbing crowd, street or office noise).Relative Excitation coefficients provides 41% relative error reductionin this test.

FIG. 7c compares WER of Relative Excitation coefficients and MFCC fornoisy speech (Artificial white noise added, SNR is about 8-12 db).Relative Excitation coefficients provides 44% relative error reductionin this test. FIGS. 7a-7c show the result of the accuracy testsconducted to compare Relative Excitation and MFC coefficients. All testsare conducted on a speaker-independent English speech recognition systemhaving a 300K sized vocabulary, 220M Trigrams, 14K HMM states and 8mixtures with diagonal covariances for each state. MFC and RelativeExcitation acoustic models are trained with the same audio corpus having180K utterances. Training and test utterances arefound-speech-utterances and may have mild or non-disturbing level ofnoise. There are more than 6000 different speakers in training corpusand 320 different speakers in test set. Embodiment 1 is used forcomputing Relative Excitation coefficients in these tests.

FIG. 8 shows frequency responses of two different high qualitymicrophones to visualize effects of channel variation. Please noticethat the deviation between 100 hz and 5 khz is 15 db for one of themwhile it is close to zero for the other. This deviation can be muchlarger for low quality microphones. Cepstral coefficients are toosensitive for these deviations in contrast to Relative Excitationcoefficients.

FIG. 9 shows comparison of MFC and Relative Excitation coefficients fortraining with noisy speech. Healthy training with noisy speech isimportant especially for using found speech data to train a speechrecognition system. Embodiment 1 is used for calculating RelativeExcitation coefficients in this test.

FIG. 10a is the Relative Excitation envelope computed from the framewhich's amplitude spectrum is drawn in FIG. 5a . Coefficients are scaled500 times.

FIG. 10b is the Relative Excitation envelope computed from the framewhich's amplitude spectrum is drawn in FIG. 5b . Coefficients are scaled500 times. FIGS. 10a-10b show Relative Excitation envelope of a cleanframe and its noisy version computed by Embodiment 2. FIGS. 10a and 10bhave a close relation with FIGS. 6a, 6b, 6c, and 6d and show the effectof noise for the embodiment 2. Please notice that FIG. 10a and FIG. 10bhave almost the same values in the region that is below 3.3 khz wherethe signal is dominant, and different values for the noise-dominantregion that corresponds the last 4 coefficients of embodiment 1explained in FIGS. 6c and 6d . Please read the noise-immunity sectionfor more information on these graphics.

FIG. 11a shows the Relative Excitation envelope of a vowel frame. Tounderstand how relative excitation method incorporates formantinformation in coefficients, please notice the formants of the spectrumand their corresponding representation on the envelope curve. Theelements of a Relative Excitation feature vector can be seen as sampledpoints of a curve as in this figure.

FIG. 11b shows the Relative Excitation envelope estimated from adeformed version of the spectrum drawn on FIG. 11a . Linear frequencyresponse curve of the deformation is also drawn. Please notice thatRelative Excitation coefficients remain stable although the spectrum isdeformed. The frequency response curve is scaled for better viewing, theminimum coefficient is 0.5 at the left side and the maximum is 2.5 atthe top of the sinusoid causing a maximum of 14 db relative deformationfor spectral component ratios. FIGS. 11 a and 11 b showchannel-independence of Relative Excitation method. Embodiment 2 is usedfor estimating Relative Excitation envelopes. Please note that Eq3 isused in Embodiment 2 for a better visualization since Eq1 inverts thepeaks. Relative Excitation coefficients are scaled 400 times.

FIG. 12 shows bandwidths of different filters. Surprisingly, relativeexcitation method uses much wider filters compared to other soundprocessing methods. Optimum 3 dB bandwidth of Relative Excitationfilters are about 160 Mels.

RELATIVE EXCITATION FEATURE EXTRACTION METHOD

Digital audio data of an utterance is divided into frames, thenfollowing 5 steps are repeated for each frame.

-   -   1—Estimate the short term amplitude spectrum.    -   2—Compute amplitudes of main components.    -   3—Compute amplitudes of comparison components.    -   4—Compute relative excitation coefficients.    -   5—Compute wide-band energy coefficients.

Delta and delta-delta (acceleration) coefficients are added to featurevectors upon completion of required frames. Mean and variancenormalization is applied for the utterance.

The main and comparison components are spectral components. Thesespectral components can be bandpass filters of a filter bank, frequencybands or bins of a frequency transform function like FFT, or points of aspectral envelope.

The Relative Excitation method is explained via frequency bands forachieving simplicity in the explanation. Embodiment 1 shows animplementation of the Relative Excitation method that utilizes frequencybands. Embodiment 2 is an example where bin amplitudes are used insteadof band amplitudes for computing Relative Excitation coefficients. Whenshorter spectral components are used as in Embodiment 2, the comparisonresults are integrated to form meaningful coefficients.

Please note that there is no step for pre-emphasis or equal-loudnesscontour weighting in contrast to MFCC and PLP, because RelativeExcitation method compensates the amplitude attenuation of spectralregions or provides regional gain-invariance for the comparisons thatare made between spectral components of the same region. Moreover, noamplitude non-linearity function (like log-compression or root cube) isrequired for estimating Relative Excitation coefficients in contrast toCepstrum or PLP.

Relative Excitation coefficients can represent any region of thespectrum or the whole spectrum without any dependency on the signal orthe spectral composition. Relative Excitation method has the ability toutilize a predetermined uniform comparison structure which depicts thecompared spectral component positions. Moreover Relative ExcitationMethod incorporates the formant information among coefficients withoutactually searching the formants. There is no need for dynamicallyselecting the spectral component positions or compared componentsaccording to formant positions, pitch frequency, harmonic positions,spectral peaks or similar properties of the acoustic signal. Currently,existing methods for formant frequency, pitch frequency and harmonicposition estimation are not reliable. Even if there were a reliablesolution, having a dynamic feature extraction structure which depends onthe signal is a heavy handicap to form consistent feature vectors. As aresult, the ability to utilize a predetermined uniform comparisonstructure;

-   -   makes the Relative Excitation method simple and reliable,    -   makes the Relative Excitation method usable for any audible        signal including human speech,    -   makes the Relative Excitation coefficients usable for speech        recognition, since consistent measurements that form a        consistent feature vector become possible. In other words,        Relative Excitation feature vectors can be utilized for training        and recognition, because the same elements of different feature        vectors bear comparable information originating from the same        measurement.

Please check FIG. 11a to see how formant and spectral peak informationis incorporated in Relative Excitation coefficients.

Computing the Amplitudes of Main Bands

The amplitude spectrum is warped according to the Mel-scale or a similarlogarithmic scale in order to adapt the frequency resolution to theproperties of the human cochlea.

Generally, 13-16 main bands are used for speech recognition. To providemore detail, the number of the main bands can be increased if memory andprocessing load is not a matter.

Main bands should be distributed evenly on the Mel-scale. But it ispossible to increase the intensity of coefficients for some regions ofthe spectrum, in contrast to MFCC and PLP.

The basilar membrane within the cochlea disperses incoming sound wavesto separate frequencies spatially. Since the basilar membrane is aphysical structure, vibrations affect a region rather than just onesmall point. The strongest vibration and stimulation occur at theresonance point. As the distance from the resonance point on basilarmembrane increases the strength of vibration decreases. This feature ofcochlea is modeled with some different filter shapes depending on theapplication.

Triangular filters of MFC, the critical band curves of PLP or gammatonefilters can be utilized, preferably with an increased bandwidth forbetter performance.

We conducted our tests with sinusoidal filters, just because of theirsimplicity and some ability to simulate the dispersion of vibrations onthe basilar membrane.

The width of the main bands has a critical importance and depends on theshape of the filter function. Basically the 3 dB bandwidth of thefrequency bands or bandpass filters must range between 120 to 240 Melson the Mel-frequency scale or equivalent range on another scale. 3 dBbandwidth is the bandwidth measured at half-power points or the pointswhere the gain is −3 dB, or 0.707 relative to peak. Decreasing thebandwidth increases the negative effects of harmonics, and increasingthe bandwidth decreases the resolution. Our experiments show that theoptimum 3 db bandwidth of main bands is approximately 160 Mels.Surprisingly, Relative excitation method uses much wider filterscompared to other sound processing methods. Please see the FIG. 12.

The width of the sinusoidal filters for main bands should range between240 to 440 units on the Mel-frequency Scale (the bandwidth measured atzero points of the sinusoidal filter frequency response). We mentionedthis bandwidth to provide simplicity in understanding the Embodiment 1.

To estimate the main band amplitudes, amplitude of each frequency bin ofa band is weighted with the corresponding filter coefficient, then thesum of the weighted amplitudes are normalized with the sum of the filtercoefficients.

Computing the Amplitudes of Comparison Bands

There must be one or more comparison bands for each main band. So, thenumber of comparison bands are equal to the number of main bands or canbe more if more than one comparison will be made for a main band.Separately computing comparison band amplitudes is not obligatory, tosave some processing power, other main bands can be used as thecomparison bands of a main band.

The distance of a comparison band from its main band on the frequencyscale has critical importance. Our experiments show that the comparisonof two frequency bands that are separated with a distance ranging from40 to 800 units on the Mel-Frequency Scale provides usable coefficients.

Having a comparison band with a distance less than 40 Mels actuallymeans trying to compare the same spectral component or vibration, so thecomparison becomes useless.

Having a large comparison distance (for example larger than 800 Mels)suffers from the increased effects of channel frequency responsevariation and regional attenuation of spectral components that makecomparison results unstable. Moreover, it causes to compare lowfrequency speech components with the high frequency speech componentsthat generally don't exist together, so the comparison becomes useless.In other words, when you increase the comparison distance too much thereis an increased possibility of comparing spectral speech components withthe noise, because vowels or consonants have spectral energiesconcentrated on definite regions of the spectrum. These issues form somemajor problems for the cepstrum method, because the DCT transform of thecepstrum method makes the whole spectrum affect all coefficients. Sincethere is no way to overcome these problems in the cepstrum method,scientists have conducted thousands of research studies to find asolution or some compensation in the frequency domain before calculatingcepstral coefficients. In contrast to cepstrum, Relative Excitationmethod gives strong control on these issues.

Just like the main bands, the width of the comparison bands has acritical importance and depends on the shape of the filter function.Basically, the 3-db bandwidth of the comparison bands must range between120 to 240 Mels. Our experiments show that the optimum 3-db comparisonbandwidth is approximately 180 Mels.

The width of the sinusoidal filters for comparison bands should rangebetween 240 to 440 units on the Mel-frequency Scale (the bandwidthmeasured between zero points of the sinusoidal filter frequencyresponse, or the bandwidth of non-zero coefficients). Again, wementioned this bandwidth to provide simplicity for understanding theEmbodiment 1.

To estimate a comparison band amplitude, amplitude of each frequency binof a band is weighted with the corresponding filter coefficient, thenthe sum of the weighted amplitudes are normalized with the sum of thefilter coefficients.

Please note that no non-linearity function (like log-compression or rootcube) is utilized.

Computing Relative Excitation Coefficients

Main band and comparison bands simulate different neuron groups on thetonotopic mapping or basilar membrane. The name “Relative Excitation”implies the relative amplitude of a main band with regards to itscomparison band or relative excitation level of a neuron group withregards to its comparison neuron group. Mathematically, RelativeExcitation coefficients are the quantitative measures of therelationship between amplitudes of two spectral components that arebeing compared.

Direct amplitude ratios of main and comparison bands or difference oflogarithmic amplitudes could be used as the comparator function. But,these approaches are vulnerable to noise and not stable enough. Wepropose two new comparator functions:

Cre=0.5−(Amb/(Amb+Acb))  (Eq1)

Cre=(Amb−Acb)/(Amb+Acb)  (Eq2)

Cre=Relative Excitation Coefficient

Amb=Main band amplitudeAcb=Comparison band amplitude

Please note that amplitudes are in the linear scale. In other words, nonon-linearity function, either logarithm nor root cube is utilized.

Both of these functions provide immunity against wide-band noise andproduce coefficients with excellent stability. Moreover, both of thecomparison functions strongly simulate the physical limits of basilarmembrane.

Human hearing covers a huge range from 0 db to 160 db. We believe,relative excitation mechanism has an important role for nonlinearhearing in such a huge range. Both of the comparison functions stronglysimulate incredible human hearing range for different sound pressurelevels and sensitivity to very weak signals.

The first comparison function (Eq1) gives coefficients between −0.5 and0.5. This function cancels out the wide-band noise that covers thecomparison distance. When the main and comparison bands have similarnoise levels, it is obvious that the coefficients move towards 0 withthe increasing noise, which causes the relative excitation range andvariance to decrease. Variance normalization on the coefficients of theutterance cancels this effect.

The second comparison function (Eq2) gives coefficients between −1and 1. This function cancels out the wide-band noise that covers thecomparison distance. When noise the level increases, the denominator ofthe function increases, and coefficients move towards 0, which in returndecreases the variance of the output. Similar to the first function,variance normalization on the coefficients of the utterance cancels thiseffect.

The following comparison function (Eq3) is basically the same as Eq1 andgives coefficients between 0 and 1. Since Eq1 inverts the peaks, we usedEq3 for preparing the data of FIG. 11 to visualize the relation betweenspectrum and Relative Excitation coefficients.

Cre=Amb/(Amb+Acb)  (Eq3)

Computing Wide-Band Energy Coefficients

Relative Excitation Coefficients are absolutely gain-invariant. Someextra coefficients are beneficial to capture the temporal energyproperties of speech-production and to compensate the exaggeratedeffects of noise when there is no signal. Solution is very simple.Logarithm of sum of 2-5 neighboring main band amplitudes are used as acoefficient.

Spectral composition and loudness are two different things also forhuman auditory perception. For example, when you increase the volume ofthe music player your perception of the spectral composition doesn'tchange, but perception of the loudness change.

Generally 4-6 wide band energy coefficients provide good results and thenumber of the coefficients can be increased if memory and processingload is not a matter.

Differences of wide-band energy coefficients can also be used asfeatures.

Channel-Independence

Different channels (microphones, electronics, compression techniques,etc.) have different frequency responses. Cepstrum method is very weakagainst these variations. On the contrary, human auditory perception isvery strong.

Here are two examples for understanding the channel-independence. First:As a sound signal is propagated from source to receiver, the amplitudesof different frequency components may be attenuated by the environmentat different rates. High-frequency components of signals are attenuatedby the environment at a higher rate than low-frequency ones. As long asthe sound is audible, this phenomenon has almost no effect onintelligibility of the speech. Second: Different frequency responses ofdifferent microphones have no or little importance on theintelligibility or our perception of the recorded speech.

Both of the examples are related to robustness against significantdeformations of the acoustic spectrum. Generally scientists believespeech-production and auditory perception have a close relation or bothof these functions support each other. So there must be a commoncharacteristic of speech production and perception that provides therobustness against spectral deformation. We explain this robustness withclose spectral component relations.

Please examine frequency response curves in FIG. 8. The point is that,the frequency response curve of a microphone has similar values forclose-frequencies, so the amplitude ratios between close frequencycomponents are preserved, but the amplitude ratios of distant componentsare distorted significantly. The frequency response deviation between100 hz and 5 khz is 15 db for one of the microphones (the continuousline of FIG. 8). But the deviation between 200 hz and 300 hz is 2 db forthe same microphone.

Now, it is obvious that relations of the close spectral components arepreserved while the amplitude relations of distant components aredistorted significantly. Let's explain how and why this property isuseful. Please check the FIG. 5a . An accustomed eye can easily see thethree formants. First one, F1 is around 500 Hz, F2 is close to 1 Khz andF3 is close to 2 Khz. One can find out each formant frequency bychecking the neighboring harmonics around the candidate frequency, notthe distant ones. So, close spectral component relations have muchimportance than the distant ones and these relations are stronglypreserved. If the amplitude of F3 were halved and F1/F3 amplitude ratiodoubled due to the frequency response of the microphone, our perceptionwouldn't change much, and similar spectral deformations are very common.But it is really difficult for a channel to cause a significantdeviation of the ratios of the close harmonic amplitudes (which identifythe formant) that results with a change at the formant position. It canbe seen that human auditory assigns much more importance to closecomponent relations than the distant component relations for theperception of the spectral composition. We believe this is the mostimportant reason of the human auditory robustness against channelfrequency response variation.

Relative Excitation method can easily simulate this property of humanauditory perception by utilizing close comparison distances, for examplea comparison distance less than 300 Mels.

Please check FIG. 11 to see how excellently Relative Excitation methodhandles the spectral deformations. FIG. 11a shows the original spectrumand its Relative Excitation coefficients. FIG. 11b shows the deformedspectrum, its Relative Excitation coefficients, and the frequencyresponse curve of the deformation. Please notice that RelativeExcitation coefficients remain stable although the spectrum issignificantly deformed.

In contrast to Relative Excitation, the Cesptrum method fails againstchannel frequency response variation. As a result when you change themicrophone of an MFC based speech recognition system after training, therecognition performance degrades. On the contrary, a speech recognitionsystem based on Relative Excitation features remains robust.Additionally, training an MFC based speech recognition system with audiorecorded from different microphones cause coarse acoustic models.

Noise-Immunity

Relative Excitation Features are excellently engineered for noiseimmunity. Our approach includes some strategies and a very importantnovel method that will redefine noise robustness.

Some Strategies and Properties Against Noise

Wide band noise is canceled due to the nature of the Relative Excitationcoefficient estimation. In addition to this, narrow band noise affectsonly one coefficient, since each Relative Excitation coefficient iscompletely independent (in contrast to cepstral coefficients). Since,Cepstrum is the inverse Discrete Fourier Transform (DFT) of the logspectrum, all of the spectral components affect each cepstralcoefficient. In other words, some noise in any spectral region affectsall of the cepstral coefficients.

Redundancy in speech provides a basis for error correction andresistance to noise for humans (Coker and Umeda 1974). RelativeExcitation features preserve redundant information that is useful fornoise-immunity. Number of Relative Excitation feature coefficients canbe increased to provide better detail and more redundant information tothe recognizer. It is possible to increase the number of the RelativeExcitation coefficients without “curse of dimensionality”. “Curse ofdimensionality” is the name of the situation where, as the dimensions ofthe feature vector are increased, the performance of the systemdecreases. Increasing number of PLP, MFC and other cepstrum basedcoefficients causes “Curse of dimensionality”.

Relative Excitation method removes the need to find a propernon-linearity function to simulate human hearing like log-compression orroot cube. Selecting a non-linearity function to simulate human hearingis a controversial subject. Mainly log-amplitude and cubic-rootrepresentations are used. Both of these functions have some drawbacks.Relative Excitation method can directly use the linear amplitude ofspectral components.

Moreover, log-compression or root cube non-linearity functions reducethe signal and noise discrimination capability of the system by makingsignal and noise numerically closer. Please see the FIGS. 7a, 7b and 7c. Noise spectrum and signal spectrum is much more distinguishable inFIG. 7b then FIG. 7c . It is obvious that utilizing the log-spectrumdeteriorates robustness against noise. In addition to this, we know thatlogarithm is not stable for small amplitudes. Despite of thesedrawbacks, the log-amplitude is inevitable for estimating MFCcoefficients.

A Novel Noise Robust Speech Recognition Technique

The short term speech spectrum almost always has signal and non-signal(or weak-signal) regions depending on the sound being produced or thestate of the articulatory organs. Noise becomes the dominant factor fornon-signal spectral regions as in FIG. 5b (the region above 3 khz).Cepstrum based methods naturally distribute the effects of the noise toall coefficients that results with coarse acoustic models, becausevariance of all coefficients are increased.

Please check FIG. 6 and notice that all of the MFC coefficients (FIG.6a-6b ) have large deviation in contrast to Relative Excitationcoefficients (FIG. 6c-6d ). Deviations of the last four RelativeExcitation coefficients (11-12-13-14) are high and this is normal (FIG.6d ). Regional gain-invariance properties of relative excitationcoefficients exaggerates the effects of the weak noise of non-signalregions (as in FIG. 6d and FIG. 10b ). This is true even for highquality speech and this is what we want.

Designing a the Relative Excitation feature extraction system that has aclose comparison distance, for example less than 300 Mels, causes aspecial circumstance which we named frequency-to-coefficient mapping,because each coefficient bear the relationship information of thespectral components located on a definite short spectral segment(between the centers of the compared bands). As an example, inEmbodiment 1, each relative excitation coefficient representsapproximately a 120 Mels wide spectral segment or frequency band, and atotal of 15 coefficients represent the whole spectrum. The term“frequency-to-coefficient mapping” indicates that the parametricrepresentations of spectral segments or frequency bands are directlymapped into coefficients of the acoustic feature vector.

Training statistical acoustic models of context dependent phone stateswith a frequency-to-coefficient mapping acoustic feature set causes thespeech recognition system to learn the signal and non-signal regions ofthe spectrum. Well trained, close comparison distance RelativeExcitation Gaussian Mixture Models have high variance elements for thesenon-signal noisy regions, since the non-signal region coefficients havehigh variation due to the infinite diversity of noise, and thespeech-signal region coefficients have relatively low variation due tothe acoustic limits of the pronunciation of the same sound. Largevariance elements of trained models cause these non-signal spectralregions to be neglected during recognition. This resembles the dynamicspectral focusing and ignorance capability of human auditory perception.This technique makes the recognizer to focus on the speech-signalexpected regions of the spectrum. Moreover, wide band energycoefficients are compatible with this paradigm to some degree, and addsextra information to the feature vector for identifying the signal andnon-signal regions.

The variances of a context dependent vowel (similar to FIG. 6d ) HMMtriphone state is copied below. The Gaussian mixture model is trainedwith relative excitation feature vectors having scaled coefficients ofEq-2. Please notice the variance of the last 4 elements which are in thenon-signal region of the context dependent vowel spectrum.

V01=107 V02=108 V03=113 VO4=159 V05=184 V06=140 V07=096 V08=111 V09=142V10=179 V11=264 V12=463 V13=491 V14=558 V15=565

For many years, scientists have been trying to cancel noise or itseffects. These efforts have reached a limited success because of thediversity of the noise. For the first time, with this invention, asolution has emerged which has capability to ignore any kind of noise(to some degree) for speech recognition systems.

It is obvious that this technique doesn't provide benefit for the noiseof spectral regions that are expected to have the speech signal. But,generally speech recognition systems use close talking microphones thatmake the speech signal much stronger than the noise, because intensityof sound is inversely proportional to the square of the distance. As aresult, we can accept that the signal components of the speech spectrumwill be generally stronger than the noise for speech recognitionsystems. For the signal regions of the spectrum, if the amplitude of thenoise is lower than the amplitude of the signal, Relative Excitationmethod excellently estimates robust coefficients that are not verydifferent from the clean version of the same signal. As long as thefrequency component of the signal is stronger than the noise, RelativeExcitation Method is very powerful for computing the accuratecoefficients. This feature also resembles the robustness of humanauditory perception.

Cepstrum method gives equal opportunity to any region of the spectrumfor affecting all of the coefficients. This is one of the most importantreasons of the failure of Cepstrum technique against noise.

We can summarize our novel approach for noise robust speech recognitionwith the following steps:

-   -   1. In the training phase, extract features that are based on the        short term spectrum of the acoustic signal in a way that each        coefficient or element of the feature vector represents only a        definite region or segment of the spectrum, so noisy spectral        components or regions affect only the corresponding        coefficients. We named this technique as        frequency-to-coefficient mapping feature extraction.    -   2. Train the acoustic models of the speech recognizer with the        features extracted in the first step. Ordinary Gaussian mixture        models can be utilized. Preferably context dependent phone        models should be used. The variances of the acoustic feature        vector elements must be estimated. The non-signal (or        weak-signal) and noise dominated spectral region coefficients        will have high variance or deviation due to the infinite        diversity of noise, in contrast, the speech-signal region        coefficients will have relatively low variance or deviation due        to the acoustic limits of the pronunciation of the same sound.        As a result, the speech signal expected regions of the spectrum        are automatically learned.    -   3. In the recognition phase, use the feature extraction        technique of the step 1 to compute the acoustic observation        vectors. Utilize an acoustic observation probability or        likelihood estimator that accounts for the variance, such as        Gaussian mixture models. Since the observation probability        estimator inversely weights the distance (difference from the        expected value) by the variance, significance of a feature        vector element in the resulting probability decreases with the        increasing variance. In other words, the high-variance        coefficients that represent the noise dominant or non-signal        regions of the spectrum have less weight in resulting        probability or distance. As a result, just like humans, the        recognizer focuses on signal expected regions of the spectrum        while noise-dominant regions are ignored.

To show the effectiveness of this method and other noise robustproperties of Relative Excitation method, we made a comparison test. MFCand Relative Excitation based speech recognition systems are trainedwith noisy audio for this test. As expected, some limited noise in thetraining audio deteriorates the MFC HMM while the Relative ExcitationHMM remains robust. Please see the FIG. 9. Both of the systems aretrained with the same audio corpus. Constant level of white noise isinjected into all utterances before training. Test utterances are clean.In this test Relative Excitation method achieves 65% error reductioncompared to MFC. This result is important especially for the speechrecognition systems that are trained with found acoustic data.

Dynamic Spectral Range

Many speech recognition systems analyze frequencies up to 7 Khz and needwideband audio. But wideband audio is not always available. Such as acall recording received from PSTN with a frequency range of 300-3300 Hz.Please think about a television broadcast having a speaker in the studioand a remote speaker connected over PSTN. In this case wideband andnarrowband audio will be in the same recording or in the same channel.Cepstrum based feature extraction method and acoustic models can'thandle this issue.

If a cepstrum based system is configured and trained for narrowbandaudio, it is possible to use the system for both audio types but theextra intelligibility provided by wideband audio will be lost, becausesignificant amount of consonants' energy remains beyond the 3.3 kHzcutoff level of the PSTN standard.

If a cepstrum based system is configured and trained for wideband audio,the system will fail to recognize narrowband audio, since blank regionsof the spectrum will affect of all the cepstral coefficients.

Frequency-to-coefficient mapping Relative Excitation coefficientsprovide the best possible solution. We can summarize this noveltechnique with the following steps:

-   -   1. Configure a Relative Excitation based speech recognizer for        wideband audio and and train it with wideband audio.    -   2. When a new utterance is received for recognition check the        upper and lower limits of the spectrum.    -   3. Adjust the acoustic probability or likelihood estimator        function to utilize only the elements of the acoustic feature        vector that are settled in the spectral range of the acoustic        observation signal. So that blank regions of the spectrum are        excluded from the acoustic probability estimation.

The correspondence of the feature vector elements to the frequency bandsor spectral regions and the independence of the feature vector elementsmake this solution possible.

Description of Embodiment 1

Framing: 16 khz PCM digital audio is divided into 25 ms frames with a8.75 ms shift. Each frame is multiplied by a hamming window.

Conversion to frequency domain: A 1024 point FFT resulting with 512frequency bins is used. Each bin frequency is converted to mel-scalewith the following pseudocode.

for BinNo=0 to 511 do Mel[BinNo]= 2595 * log10(1 +(BinNo*(SamplingFrq/1024)) / 700 );

Main Bands: 15 main bands are used. Main bands are evenly distributed onthe Mel-Frequency scale. Each main band has 160 Mels distance to thenext. The first band's center frequency is 290 Mel (205 hz). The lastband's center frequency is 2530 Mel (5907 Hz). The width of the mainband sinusoidal filters is 320 Mels (the bandwidth measured between zeropoints of the sinusoidal filter frequency response). This means actualsinusoidal filter width on the linear scale depends on the centerfrequency of the main band. Each main band has it's own sinusoidalfilter. Here is the pseudocode for calculating main band centers andsinusoidal filter coefficients:

for BandNo=0 to 14 do begin  MainBand[BandNo].Center= 290 + 160*BandNo; CoeffCnt= 0;  leftCoeffCnt= 0;  RightCoeffCnt= 0;  for BinNo= 0 to 511do  begin   if (Mel[BinNo] >= MainBand[BandNo].Center−160) and   (Mel[BinNo] < MainBand[BandNo].Center) then   begin   MainBand[BandNo].Bins[CoeffCnt]= BinNo;    inc(CoeffCnt);   inc(leftCoeffCnt);   end;   if (Mel[BinNo] >=MainBand[BandNo].Center) and    (Mel[BinNo] <(MainBand[BandNo].Center+160)) then   begin   MainBand[BandNo].Bins[CoeffCnt]= BinNo;    inc(CoeffCnt);   inc(RightCoeffCnt);   end;  end;  MainBand[BandNo].CoeffCnt=CoeffCnt;  for CoefNo=0 to leftCoeffCnt−1 do    MainBand[BandNo].FilterCoeff[CoefNo] = sin(pi*CoefNo/(leftCoeffCnt*2));  for CoefNo=0 to RightCoeffCnt−1 do    MainBand[BandNo].FilterCoeff[leftCoeffCnt+CoefNo]=      sin(pi/2 −pi*CoefNo/(RightCoeffCnt*2)); end;

Here is pseudocode for calculating main band amplitudes.

for BandNo=0 to 14 do begin  MainBandAmp[BandNo]= 0;  SumCoeff=0;  forCoeffNo=0 to MainBand[BandNo].CoeffCnt−1 do  begin  MainBandAmp[BandNo]= MainBandAmp[BandNo] +   MainBand[BandNo].FilterCoeff[CoeffNo] *    spectrum[MainBand[BandNo].Bins[CoeffNo] ];   SumCoeff= SumCoeff +MainBand[BandNo].FilterCoeff[CoeffNo];  end;  MainBandAmp[BandNo]=MainBandAmp[BandNo]/SumCoeff; end;

“spectrum[ ]” is the FFT amplitude spectrum for the windowed frame.

Comparison Bands: One comparison band is used for each main band. Eachcomparison band has a center frequency that is 120 Mels higher than itsmain band center. The width of a comparison band sinusoidal filter is360 Mels (the bandwidth measured between zero points of the sinusoidalfilter frequency response). Actual sinusoidal filter width on the linearscale depends on the center frequency of the comparison band. Eachcomparison band has its own sinusoidal filter. Here is the pseudocodefor calculating comparison band centers and sinusoidal filtercoefficients.

for BandNo=0 to 14 do begin  ComparisonBand[BandNo].Center=MainBand[BandNo].Center + 120;  CoeffCnt= 0;  leftCoeffCnt= 0; RightCoeffCnt= 0;  for BinNo= 0 to 511 do  begin   if (Mel[BinNo] >=ComparisonBand[BandNo].Center−180) and     (Mel[BinNo] <ComparisonBand[BandNo].Center) then   begin   ComparisonBand[BandNo].Bins[CoeffCnt]= BinNo;    inc(CoeffCnt);   inc(leftCoeffCnt);   end;   if (Mel[BinNo] >=ComparisonBand[BandNo].Center) and     (Mel[BinNo] <(ComparisonBand[BandNo].Center+180)) then   begin   ComparisonBand[BandNo].Bins[CoeffCnt]= BinNo;    inc(CoeffCnt);   inc(RightCoeffCnt);   end;  end;  ComparisonBand[BandNo].CoeffCnt=CoeffCnt;  for CoefNo=0 to leftCoeffCnt−1 do   ComparisonBand[BandNo].FilterCoeff[CoefNo] =      sin(pi*CoefNo/(leftCoeffCnt*2));  for CoefNo=0 to RightCoeffCnt−1 do    ComparisonBand[BandNo].FilterCoeff[leftCoeffCnt+CoefNo]=     sin(pi/2 − pi*CoefNo/(RightCoeffCnt*2)); end;

Here is the pseudocode for calculating comparison band amplitudes:

for BandNo=0 to 14 do begin  ComparisonBandAmp[BandNo]= 0;  SumCoeff= 0; for CoeffNo= 0 to ComparisonBand[BandNo].CoeffCnt−1 do  begin  ComparisonBandAmp[BandNo]= ComparisonBandAmp[BandNo] +   ComparisonBand[BandNo].FilterCoeff[CoeffNo] *    spectrum[ComparisonBand[BandNo].Bins[CoeffNo] ];   SumCoeff= SumCoeff +ComparisonBand[BandNo].FilterCoeff   [CoeffNo];  end; ComparisonBandAmp[BandNo]= ComparisonBandAmp[BandNo]/  SumCoeff; end;

Relative Excitation Coefficients: 15 relative excitation coefficientsare calculated with the Eq-2. Here is the pseudocode for calculating therelative excitation coefficients:

for BandNo=0 to 14 do begin  RelexCoeff[BandNo]=(MainBandAmp[BankNo] − ComparisonBandAmp[BandNo]) /   (MainBandAmp[BandNo] +ComparisonBandAmp[BandNo]) ; end;

Wide-Band Energy Coefficients: 5 wide-band energy coefficients arecomputed. Each coefficient is calculated from 3 neighboring main bandamplitudes:

for WideBandNo=0 to 4 do begin  WdBndEngCoeff[WideBandNo]=ln(MainBandAmp[WideBandNo*3] +   MainBandAmp[WideBandNo*3+1] +  MainBandAmp[WideBandNo*3+2] ) ; end;

Relative Excitation Coefficients and Wide-Band Energy Coefficients arecombined on a 20 dimensional single vector. For each frame, delta andacceleration coefficients are added to form a 60-dimensional acousticfeature vector. Mean and variance normalization is applied to each frameof the utterance.

Description of Embodiment 2

In this embodiment bins of an FFT transform are used as spectralcomponents. Although Embodiment 2 and Embodiment 1 seems to bedifferent, they are very similar. Both of the Embodiments simulaterelative excitation levels of different neuron groups. In Embodiment 2comparison results are integrated instead of integrating the binamplitudes located in a frequency band prior to comparison as inEmbodiment 1. The method explained in this embodiment can also be usedfor narrow spectral component's instead of frequency bins. Embodiment 2provides similar accuracy compared to Embodiment-1 with the capabilityto control the weight of each frequency bin or the weight of acomparison in the resulting coefficient, but requires more computation.Having the capability to control the weight of each frequency bin orcomparison provides extensive possibilities for processing the signal inthe frequency domain.

Frame blocking and frequency domain conversion is the same withEmbodiment-1. Then, these steps are applied:

1—Estimate wide band energy coefficients.2—Estimate relative excitation spectrum.3—Estimate relative excitation envelope.4—Estimate relative excitation coefficients by resampling the relativeexcitation envelope.

Estimating wide band energy coefficients is the same as in Embodiment-1.

Estimating relative excitation spectrum comprises novel techniques whichare very powerful and very interesting.

Amplitude of each bin is compared with following bins that are placed ina certain spectral range. Each comparison produces a coefficient that isweighted according to it's distance and compared bin's amplitude. Sum ofweighted coefficients of a bin is normalized by sum of weights to form abin of the relative excitation spectrum.

Weighting according to compared bin's distance is applied because wewant distant comparisons have less weight and try to limit thecomparison in a certain band that simulates a neuron group on tonotopicmapping.

Weighting according to compared bin's amplitude is applied to increasethe weight of comparison coefficients of high amplitude bins. Because weknow resonance points on basilar membrane forms actual stimulation, andin addition to this, low amplitude bins generally represent noise.

Weighting according to compared bin's amplitude is optional. Thistechnique provides extra robustness against wide-band noise like whitenoise or crowd noise but causes extra sensitivity for noises havingsharp peaks in the spectrum such as resonance point of a musicalinstrument's sound. In our tests, amplitude weighting for a specific binis extended to include bin amplitudes of possible a few harmonicfrequencies and better results against noise are achieved.

Eq-1 is utilized in this embodiment and relative excitation spectrum isestimated with following pseudocode:

for BinNo=0 to BinCount−9 do begin  CompLen = 20 + BinNo div 8;  Last =BinNo + CompLen;  if Last>BinCount−1 then Last=BinCount−1;  TotalK= 0; ExcitationSpect[BinNo]=0;  for I= BinNo to Last do  begin   K =spectrum[I] * (1− (I−BinNo) / CompLen);   ExcitationSpect[BinNo]=ExcitationSpect[BinNo] +    K *spectrum[BinNo]/(spectrum[BinNo]+spectrum[I]);   TotalK= TotalK+K;  end; ExcitationSpect[BinNo]= ExcitationSpect[BinNo]/TotalK; end;

The “CompLen=20+BinNo div 8;” line is for calculating the range ofcomparison and simply simulates constant bandwidth in logarithmic scalethat increases in linear scale. This comparison range equals 312 hz at200 hz and 812 hz and 4 Khz.

The “K=spectrum[I]*(1−(I−BinNo)/CompLen)” function is used for smoothlytapering the affect of distant bins and weighting according to amplitudeof compared bin. “spectrum[I]” is the amplitude of the compared bin andthis value can be extended to have the amplitudes of a few possibleharmonics of the compared bin as“(spectrum[I]+spectrum[PossibleHarmonic1]+spectrum[PossibleHarmonic2])”As an alternative for bin-to-bin comparison, amplitude of each bin orspectral component can directly be compared to the amplitude of acomparison band instead of the bins or the spectral components locatedin the comparison band that can be named as bin-to-band comparison.

Estimating relative excitation envelope is simple. Each bin of relativeexcitation spectrum is recalculated utilizing amplitudes of neighboringbins in a certain range. This range should be adjusted to cover at leastone harmonic of high pitched human speech.

for BinNo=0 to BinCount−1 do begin  First = BinNo − 16;  Last = BinNo +16;  if First<1 then First= 1;  if Last>BinCount−1 then Last=BinCount−1; TotalK= 0;  ExcEnvelope[BinNo]=0;  for I=first to Last do  begin   K =spectrum[I] * power(0.8, abs(I−BinNo)) ;   ExcEnvelope[BinNo]=ExcEnvelope[BinNo] + K * ExcitationSpect[I];   TotalK= TotalK+K;  end; ExcEnvelope[BinNo]= ExcEnvelope[BinNo]/TotalK; end;

“K=spectrum[I]*power(0.8, abs(I−BinNo))” is for estimating a weightcoefficient that depends on the distance and the amplitude of theweighted bin. Main band sinusoidal filters are directly used on therelative excitation envelope to calculate relative excitationcoefficients. Here is the pseudocode:

for BandNo=0 to 14 do begin  RelexCoeff[BandNo]= 0;  SumCoeff=0;  forCoeffNo=0 to MainBand[BandNo].CoeffCnt−1 do  begin   RelexCoeff[BandNo]=RelexCoeff[BandNo] +    MainBand[BandNo].FilterCoeff[CoeffNo] *   ExcEnvelope[ MainBand[BandNo].Bins[CoeffNo] ];   SumCoeff= SumCoeff +MainBand[BandNo].FilterCoeff[CoeffNo];  end;  RelexCoeff[BandNo]=RelexCoeff[BandNo]/SumCoeff; end;

Relative excitation coefficients and wide-band energy coefficients arecombined on a 20 dimensional single vector. For each frame, delta andacceleration coefficients are added to form a 60-dimensional acousticfeature vector. Mean and variance normalization is applied to each frameof the utterance.

1. A method for computer acoustic recognition, the method comprising:estimating, using a processor, amplitudes of frequency bands of aspectrum of a received acoustic signal; computing, using a function ofan amplitude comparator, coefficients as acoustic features of thefrequency bands, wherein: the coefficients are quantitative measures ofa relationship between amplitudes of frequency bands compared by thecomparator; center values of compared frequency bands are separated by adistance less than 800 units on the Mel-frequency scale or less than anequivalent distance on a scale other than the Mel-frequency scale; a 3dB bandwidth of the frequency bands range between 120 and 240 Mels onthe Mel-frequency scale or an equivalent range on another scale; the 3dB bandwidth is one of measured at half-power points, or is the pointswhere the gain is −3 dB, or 0.707 relative to peak; using the estimatedcoefficients as acoustic features in a computer acoustic recognitionsystem; and performing acoustic recognition using the computer acousticrecognition system.
 2. A method for computer acoustic recognition,comprising: estimating, using a processor, amplitudes of frequency bandsof a spectrum of a received acoustic signal; computing, using a functionof an amplitude comparator, first coefficients as acoustic features ofthe frequency bands, wherein: the first coefficients are quantitativemeasures of the relationship between amplitudes of frequency bandscompared by the comparator; center values of compared frequency bandsare separated by a distance less than 800 units on the Mel-frequencyscale or less than an equivalent distance on a scale other than theMel-frequency scale; a 3 dB bandwidth of the compared frequency bands isshorter than 120 Mels on the Mel-frequency scale or an equivalent rangeon another scale; the 3 dB bandwidth is one of measured at half-powerpoints, or is the points where the gain is −3 dB, or 0.707 relative topeak; integrating, using an integrator, the results of the amplitudecomparator to obtain second coefficients, wherein the secondcoefficients are quantitative measures of the relationship betweenamplitudes of frequency bands of the spectrum of the received acousticsignal, each frequency band having a 3 dB bandwidth ranging between 120and 240 Mels on the Mel-frequency scale or an equivalent range onanother scale; using the obtained second coefficients as acousticfeatures in a computer acoustic recognition system; and performingacoustic recognition using the computer acoustic recognition system. 3.The method as defined in claim 1, wherein the center values of comparedfrequency bands are separated by a distance less than 300 units on theMel-frequency scale or an equivalent range on another scale.
 4. Themethod as defined in claim 1, further comprising combining the estimatedcoefficients with wide band energy coefficients or their differences toobtain an acoustic feature vector, wherein a said wide-band energycoefficient is defined as a coefficient that represents a logarithmicamplitude of a said frequency band.
 5. The method as defined in claim 1,wherein the coefficients are computed by utilizing a predetermineduniform comparison structure that is one of (a) not dependent on theacoustic signal, or (b) used for any acoustic signal.
 6. The method asdefined in claim 1, further comprising: comparing two frequency bands,wherein the function of the amplitude comparator is one of the ratio ofthe difference of compared frequency band amplitudes to the sum of both,or the ratio of one of the compared frequency band amplitudes to the sumof both.
 7. A method for computer acoustic recognition, the methodcomprising: estimating, using a processor, the bin amplitudes of afrequency transform function of a received acoustic signal; andcomputing, using a function of an amplitude comparator, coefficientswhich are quantitative measures of the relationship between the binamplitudes that are compared, the computing comprising, for each of aplurality of bins: comparing the bin amplitude to the respectiveamplitude of each of a plurality of different bins that are placed in apredetermined spectral range or frequency band; integrating the resultsof the comparisons made for the bin with a plurality of bins that lie ina band to form a coefficient specific to the compared bin; the computingfurther comprising integrating coefficients of neighboring bins that areplaced in a certain frequency band to form an acoustic featurecoefficient; using the acoustic feature coefficients as acousticfeatures in a computer acoustic recognition system; and performingacoustic recognition using the computer acoustic recognition system. 8.The method as defined in claim 7, wherein the acoustic featurecoefficients are quantitative measure of a relationship betweenamplitudes of frequency bands of a spectrum of the received acousticsignal, each frequency band having a 3 dB bandwidth ranging between 120and 240 Mels on the Mel-frequency scale or an equivalent range onanother scale.
 9. The method as defined in claim 7, further comprisingweighting of the comparison results.
 10. The method as defined in claim9, wherein the weighting is performed according to one of the amplitude,the distance of the compared bins, or amplitudes of harmonics of thecompared bins.
 11. The method as defined in claim 7, further comprising:avoiding long distance spectral component comparisons; and selecting thecompared bins to be in the same spectral region which has a bandwidthless than 400 Mels or less than equivalent in another scale.
 12. Amethod for computer acoustic recognition, the method comprising:estimating, using a processor, the bin amplitudes of a frequencytransform function of a received acoustic signal; and computing, using afunction of an amplitude comparator, coefficients which are quantitativemeasures of the relationship between the bin amplitudes that arecompared, the computing comprising, for each of a plurality of bins:comparing the bin amplitude to the respective amplitude of at least onefrequency band that is placed in a predetermined spectral range; thecomputing further comprising integrating coefficients of neighboringbins that are placed in a certain frequency band to form an acousticfeature coefficient; using the acoustic feature coefficients as acousticfeatures in a computer acoustic recognition system; and performingacoustic recognition using the computer acoustic recognition system. 13.The method as defined in claim 12, wherein the acoustic featurecoefficients are quantitative measure of a relationship betweenamplitudes of frequency bands of a spectrum of the received acousticsignal, each frequency band having a 3 dB bandwidth ranging between 120and 240 Mels on the Mel-frequency scale or an equivalent range onanother scale.
 14. The method as defined in claim 12, further comprisingweighting of the comparison results.
 15. The method as defined in claim14, wherein the weighting is performed according to one or more of theamplitude of the compared bin, the amplitude of the frequency band towhich the bin is compared, the distance between the compared bin andfrequency band, amplitudes of harmonics of the compared bin andamplitudes of harmonics of the frequency band to which the bin iscompared.
 16. The method as defined in claim 12, further comprising:avoiding long distance spectral component comparisons; and selecting thecompared bin and frequency band to be in the same spectral region whichhas a bandwidth less than 400 Mels or less than equivalent in anotherscale.
 17. The method of claim 1, wherein the coefficients extracted asacoustic features are used in the computer acoustic recognition systemfor one of sound recognition or training of acoustic models.
 18. Themethod as defined in claim 2, wherein the center value of respectivefrequency bands of compared frequency bands are separated by a distanceless than 300 units on the Mel-frequency scale or an equivalent range onanother scale.
 19. The method as defined in claim 2, further comprisingadding wide band energy coefficients or their differences to an acousticfeature vector, wherein a said wide-band energy coefficient is definedas a coefficient that represents a logarithmic amplitude of a saidfrequency band.
 20. The method as defined in claim 2, wherein thecoefficients are computed by utilizing a predetermined uniformcomparison structure that is one of (a) not dependent on the acousticsignal, or (b) used for any acoustic signal.
 21. The method of claim 2,wherein the coefficients are used in the computer acoustic recognitionsystem for one of sound recognition or training of acoustic models. 22.The method of claim 7, wherein the acoustic feature coefficients areused in the computer acoustic recognition system.
 23. The method ofclaim 12, wherein the acoustic feature coefficients are used in thecomputer acoustic recognition system.
 24. The method of claim 1, furthercomprising using the coefficients extracted as acoustic features forunderstanding and simulating human auditory perception.
 25. The methodof claim 2, further comprising using the coefficients for understandingand simulating human auditory perception.
 26. The method of claim 7,further comprising using the acoustic feature coefficients forunderstanding and simulating human auditory perception.
 27. The methodof claim 12, further comprising using the acoustic feature coefficientsfor understanding and simulating human auditory perception.